Phone line:  You still need to pay for your telephone line and calls (and of course a computer with a modem).

Phone number:  Local access phone numbers may or may not be available in your area.

Advertisements:  A program displaying advertisements needs to be installed (only once) and run while you're online.  The program usually runs at the top or the bottom of your screen.  If you're already using an advertisement software (most likely from a paid-to-surf service like AllAdvantage.com), this may crowd your desktop.  Running your operating systems in a high resolution with a large monitor would certainly help.

Privacy:  They collect certain information about you and your web surfing pattern.  Please read their privacy policy.  Personally, I am not too much concerned about this partly because almost all web sites appear to collect such information through cookies anyway whether you use a free or non-free ISP. 

Network:  Most free ISP's use a 3rd party network to provide access numbers.  If there's a temporary network or access number problem in your area, having another free ISP's with the same network may not work as a backup.  Looking at  whom a free ISP is powered by is one way to guess the network.

Business:  Just like any other industry, the free ISP industry appears to be dynamic.  You may see service changes (from full-free to limited-free service), bankruptcies and mergers.  Fortunately, almost of them notify you in advance for any changes and that a departing ISP usually refers you to an another free ISP.

Totally Free?:  Some ISP's advertise their services as free, but when you go to their sites, you may read certain conditions such as "free for 90 days" and "one-time setup fee required".  Please don't overlook the conditions in fine letters in their sites before you click "I Agree".